1. Field of the Invention
The present invention relates to a common mode filter and a method of manufacturing the same, and more particularly, to a common mode filter including a ferrite core provided in the center of an insulating layer and a lower substrate and/or an upper substrate by penetrating the insulating layer and the lower substrate and/or the upper substrate, and a method of manufacturing the same.
2. Description of the Related Art
In recent times, there is a need for high transmission speed due to system configuration and increases in data capacity. As a high speed transmission method, a differential signaling method is mainly used.
In general, when converting a signal into a radio frequency in order to increase transmission speed, undesired electromagnetic waves (that is, noise) are generated according to conversion of the signal into a radio frequency so that the signal and the noise are overlapped with each other.
Accordingly, common mode noise is generated due to unbalance between high speed differential signal lines (that is, two signal lines).
A common mode filter is mainly used to remove this common mode noise. The common mode filter is an EMI filter which is mainly applied to the high differential signal line.
The common mode noise is noise generated in the differential signal line, and the common mode filter removes such noise, which can not be removed by an existing EMI filter. The common mode filter contributes to improvement of antenna characteristics of mobile phones or EMI characteristics of home appliances.
However, when more data are communicated between a main device and a peripheral device over a GHz radio frequency band, as described above, it is difficult to provide smooth data processing due to signal delay and other obstacles.
Especially, in various port to port connection of communication, video, and audio signal lines such as digital TVs, the above-described internal signal line delay and transmission/reception distortion more frequently occur.
In order to solve the above problem, existing EMI prevention parts (for example, common mode filter) are manufactured in coil type or stack type, but the coil type or stack type EMI prevention parts are applied only to specific portions or large-area circuit boards since they have a large chip part size and poor electrical characteristics.
Moreover, in these days, since electronic products have been developed to have slim, miniaturized, complex, and multifunctional characteristics, EMI prevention parts that meet these characteristics are on the rise. Although the coil type or stack type EMI prevention parts, which correspond to the slim and miniaturized electronic products, have been manufactured, since there is a limitation in forming a complex internal circuit in a small area, recently, there is a need for manufacture of a thin film type common mode filter.
FIG. 1 is an exploded perspective view of a conventional common mode filter disclosed in Japanese Patent Application Laid-Open No. 2002-203718. The conventional common mode filter includes magnetic substrates 110A and 110B and non-magnetic insulating layers 120A to 120D.
Spiral coil patterns 130, 140, 150, and 160 are formed on the insulating layers 120A to 120D. The insulating layers 120A to 120D are laminated to form a non-magnetic insulator block 120. The coil patterns 130, 140, 150, and 160 are embedded in the insulator block 120, and the insulator block 120 is inserted between the magnetic substrates 110A and 110b to form the common mode filter. The coil patterns 130, 140, 150, and 160 form two coils, and terminals of each coil are electrically connected to external electrodes.
In this conventional thin film type common mode filter, adhesive strength of the external electrode to the insulator block 120 is low due to a decrease in area of the external electrode. Therefore, when being mounted to mobile electronic devices, reliability may be deteriorated.
Meanwhile, when looking into a conventional method of manufacturing a common mode filter, a green sheet is manufactured by mixing ferrite powder, a solvent, a binder, and other dispersants to prepare the mixture into a slurry or paste form and performing a die casting or printing process. Next, the manufactured green sheets are laminated with a desired thickness and sintered. In some cases, sintering is performed in a state in which a predetermined weight is applied.
At this time, when looking into the sintered substrate, there is a difference in thickness between an outer portion and a center portion of the substrate and warpage of the substrate occurs, and accordingly, the substrate is not suitable for being used as ferrite substrates for EMI and EMC such as common mode filter products since it is broken or cracked by a slight impact.
And, there is also a local difference in sintered density of the sintered substrate and a gap is generated in the non-sintered portion so that an internal gap or corrosion is caused by permeation of a chemical solution used in a photolithography process.
The thin film type common mode filter forms an internal conductor pattern on the sintered ferrite substrate by a photolithography process and forms a ferrite resin layer, which is made of a mixture of ferrite and a resin, on the substrate. At this time, the ferrite resin layer has low magnetic permeability and Q values compared to the sintered ferrite substrate and thus problems such as reduction of impedance are caused in real chip characteristics.
And, in the process of laminating the ferrite resin layer, since an outer form of the ferrite substrate is deformed or strength of the ferrite substrate is reduced, bad dimensions and adhesion with external electrodes are becoming problems.